var.visit

Apply a visitor pattern to this value, dispatching to the appropriate handler based on type.

This provides type-safe dispatch without manual switch on Type, similar to std.variant.visit. Handlers are provided as template arguments and can be lambdas, function pointers, or delegates.

Handler Selection:

• Handlers accepting specific types (bool, long, double, string, var[], var[string]) are matched first.
• A handler accepting var serves as a catch-all fallback.
• If no matching handler is found for the current type, returns the default value of the return type.

Type Mapping:

• NULL: matched by typeof(null) handler or fallback
• BOOL: matched by bool handler
• Integer types (BYTE..ULONG): matched by long handler (widened). ULONG values above long.max are matched by a ulong handler first; if only a long handler exists it sees the (wrapped) value through D's implicit conversion.
• Floating types (FLOAT, DOUBLE, REAL): matched by double handler (widened)
• STRING: matched by string handler
• ARRAY: matched by var[] handler
• OBJECT: matched by var[string] handler
• Character types (CHAR, WCHAR, DCHAR): matched by dchar handler (widened)

Fallback normalization (documented behavior, intentional): the var-typed fallback does not receive the original receiver. It sees a normalized reboxed value following the same widening as typed handlers: narrow integer tags become LONG (a BYTE receiver arrives as LONG), FLOAT becomes DOUBLE, and characters may arrive reboxed after typed dispatch. A fallback handler therefore cannot inspect the receiver's original narrow tag and may not compare equal to the original under exact-tag equality. visitNogc shares the typed-handler widening. Because the fallback value is an independent copy (deep for const receivers), mutating it cannot affect the receiver.

var v = 42;
string result = v.visit!(
   (long n) => "integer: " ~ n.to!string,
   (string s) => "string: " ~ s,
   (var fallback) => "other: " ~ fallback.toString()
);
assert(result == "integer: 42");

var s = "hello";
auto r = s.visit!(
   (long n) => n * 2,
   (string str) => cast(long)str.length
);
assert(r == 5); // length of "hello"